Sounds like an OK default, especially since the "better" (in your opinion) way can be achieved by just adding "Don't try to keep backwards compatibility with old code" somewhere in your reusable system prompt.

It's mostly useful when you work a lot with "legacy code" and can't just remove things willy nilly. Maybe that sort of coding is over-represented in the datasets, as it tends to be pretty common in (typically conservative) larger companies.

You will get better results if you reset the code changes, tweak the prompt with new guidelines (e.g. don’t do X), and then run it again in a fresh chat.

The less cruft and red herrings in the context, the better. And likewise with including key info, technical preferences, and guidelines. The model can’t read our minds, although sometimes we wish so :)

There are lots of simple tricks to make it easier for the model to provide a higher quality result.

Using these things effectively is definitely an complex skill set.